Field of the Invention
The invention relates to inverter, notably for an X-ray generator, comprising a series resonant circuit and at least two push-pull connected electronic switches which are traversed by a respective halfwave of the resonant-circuit current and via which the series resonant circuit can be coupled to a d.c. source.
Inverters of this kind are known (DE OS 30 46 413; DE-OS 30 46 767 and DE-OS 32 18 535 the latter corresponding to U.S. Pat. No. 4,514,295). The electronic switches thereof are conductive during a respective halfwave and the electric power that can be generated by such an inverter device is greater as the halfwaves traversing the electronic switches in push-pull succeed one another more closely.
In the known inverters the electronic switches are formed by thyristors which are connected to the d.c. source in the forward direction, a diode being connected to each of said thyristors in an anti-parallel configuration. In electronic switches of this kind it is not possible for the halfwaves to flow through the push-pull connected switches in direct succession, as appears from FIG. 1 which shows the variation in time of the resonant-circuit current in the known inverter device. In this respect it is assumed that a thyristor is conductive during one positive halfwave. After the current passes through zero, the diode connected parallel to the thyristor first takes up the kickback current and the triggered thyristor ceases to conduct. Due to the power output to the user the negative halfwave of the kickback current (denoted partly by a broken line in FIG. 1) has an amplitude which is lower than that of the positive halfwave.
The push-pull controlled thyristor may be triggered only when the so-called recovery time (tq) for the previously conductive thyristor has elapsed, Which recovery time is necessary for the decomposition of the charge carriers. As is disclosed in DE-OS 30 46 767, the trigger pulse can be generated by means of a delay circuit which is started by the zero crossing of the current. For recognition of the zero crossing of the current there is usually provided a comparator circuit whose input receives a value which is proportional to the instantaneous resonant circuit current and which is activated as soon as the resonant circuit current has dropped below a value i.sub.s. This value i.sub.s must be large enough to prevent starting of the delay circuit by interference signals superposed on the measuring signal, so that premature generation of the trigger pulse in given circumstances is prevented. Between the extinguishing of the one thyristor and the triggering of the next one, therefore, there should elapse at least a period of time which corresponds to the sum of the recovery time and the period of time within which the kickback current reaches the value i.sub.s after the zero crossing. In the case of a particularly large load or a particularly high power output it may even occur that the kickback current does not reach the value i.sub.s, so that the trigger pulse activating a thyrister during the next halfwave cannot be generated. This causes a collapse of the voltage or current for the load.